Tractor-trailers and other vehicles are typically equipped with brake actuators for engaging or disengaging the foundation brakes (hereafter referred to as the “brakes”). The more common among these actuators are pneumatic actuators, which are also known as air brakes. Such pneumatic brake actuators apply air to a service chamber, which moves a diaphragm connected to a push rod causing the push rod to advance which in turn causes an attached brake to actuate or engage. The push rod is connected to the brake via a link. On the other hand, removing air from the service chamber causes the push rod to retract which in-turn causes the attached brake to move back to its non-actuated position or disengage. Most brakes also have an emergency chamber attached to the service chamber, which is operable to move the push rod if and when the air system fails. To this end, a high strength power spring is typically incorporated into the emergency chamber to actuate the push rod. This spring is also typically used to actuate the push rod when the vehicle is parked.
The push rod within a brake actuator has a predetermined range of possible movement, which is commonly referred to as the “stroke movement.” Those skilled in the art may also refer to the stroke movement as a “brake stroke” or simply as a “stroke.” Because the brake is associated with the push rod, a brake may also be referred to as having a stroke movement. The stroke movement required to fully actuate a brake must be carefully monitored because it can indicate whether the brake system is functioning properly or not. For example, an excessive amount of stroke required for the brake to fully engage a wheel can indicate several faults in the brake system. Such excessive stroke movement is usually referred to as an “overstroke” condition. Typically, an overstroke condition is caused by the lining of a brake wearing or disintegrating due to repeated use. Overstroke condition can also be caused by the link connecting the push rod to the brake bending, loosening, or experiencing excessive wear. A combination of these conditions may also cause an overstroke condition.
Carefully monitoring the stroke movement can also lead to the discovery of other brake problems besides an overstroke condition. For example, a push rod may not return to its non-actuated position or its fully retracted position even after the brake pedal has been released. This is termed a “dragging brake,” and is capable of creating sufficient heat as to cause a fire due to the unwanted friction between the brake and the wheel over a prolonged period of time. Furthermore, when the operator applies the brake pedal the push rods may not move at all to engage the brake. This is termed a brake failure or non-actuation.
Currently, there are several conventional methods of monitoring the appropriateness of a stroke movement. Determining whether excessive stroke movement or an overstroke condition exists is dependent upon the designed stroke movement, or rated stroke movement, of the brake actuator which is well known to the manufacturers of the actuators. Some brake actuators include a visual indicator of a push rod experiencing excessive stroke movement, or approaching an overstroke condition. The visual indicator may be as simple as a band of bright color on the push rod, which should not extend or be visible outside of the brake chamber. An operator who sees that brightly colored band is provided with an indication that the push rod is exceeding, or may soon exceed, its designed stroke movement. Such systems have proven quite beneficial in early detection of brake problems. However, such visual systems require the operator to physically check each brake station beneath the vehicle to see the indicators of excessive push rod movement. Such systems are not convenient, require too much time and do not provide real-time information about the condition of the brakes to an operator.
Another example of a conventional vehicle brake monitor is described in U.S. Pat. No. 6,255,941 issued to Osterman et al. (hereafter referred to as the “'941 patent”). The brake monitoring system of the '941 patent includes a Hall-effect sensor mounted within the housing of a brake actuator, and a magnet with adjacent north and south magnetic fields mounted to the push rod. The sensor is used to track the movement of the push rod by sensing the movement of the attached magnet relative to the sensor's own location. Each magnet has a specific magnetic profile from its north pole to its south pole and the Hall effect sensor can determine the position of the magnet by sensing the changing magnetic field as the magnet moves. The Hall-effect sensor of the '941 patent is a ratiometric linear Hall-effect transistor, wherein the output voltage of the sensor signal is proportional to the input signal voltage and the applied magnetic field. Thus, when push rod and the associated magnet are in the non-actuated position the Hall-effect sensor senses no magnetic field and yields an output signal voltage equal to the input voltage signal which is 2.5 volts in the '941 patent.
In the magnet of the '941 patent the transition point where the two adjacent north and south fields come together has no magnetic field at all. Therefore, if the transition point is in front of the sensor the sensor senses no magnetic field at all, similar to when the magnet is in the non-actuated position. In the actuator of the '941 patent the transition point is located so as to correspond to the brake being in an overstroke condition. Thus, the output voltage of the sensor signal of the '941 patent indicates 2.5 volts for both an overstroke condition as well as a non-actuated brake condition. Reading the sensor signal of the '941 patent alone cannot indicate the difference between overstroke condition and a non-actuated brake condition. Likewise, ambiguity of the sensor signal affects the ability of a monitoring system to detect other brake conditions.
Therefore, there is need for a vehicle brake monitoring system that can accurately monitor the stroke movement of a push rod and use the resulting data to indicate the condition of a brake. There is further need for a vehicle brake monitoring system that can provide the operator of a vehicle with real-time information indicative of the performance condition of the vehicle's brakes.